This invention relates to catalyst systems useful for polymerizing alpha-olefins and particularly relates to a supported titanium halide catalyst component containing a specific class of hindered aromatic esters.
Magnesium-containing supported titanium halide-based alpha-olefin polymerization catalyst components are now well known in the art. Typically, these catalysts are recognized for their performance based on activity and stereospecificity. However, commercial olefin polymerization, especially gas-phase alpha-olefin polymerization, requires additional catalyst attributes for economical large-scale operation. Specifically, polymer morphology, typically dependent upon catalyst morphology, many times is critical. Included in good polymer morphology is uniformity of particle size and shape, resistance to attrition and an acceptably high bulk density. Minimization of very small particles (fines) typically is very important, especially in gas-phase polymerization to avoid transfer or recycle line pluggage. Very large particles also must be avoided to minimize formation of lumps and strings in the reactor. It has been found that usual modification of conventional supported catalysts to optimize morphology typically sacrifices such catalysts original activity and stereospecificity.
One catalyst system which exemplifies the activity/morphology dilemma is prepared by chlorination and precipitation of a hydrocarbon-soluble alkyl magnesium composition (which also may contain an aluminum alkyl compound) to a nearly spherical, uniform support. An olefin polymerization catalyst component is formed from such support by treatment with titanium tetrachloride and a suitable Lewis base. It has been found that a di-n-butylmagnesium/triethylaluminum complex reacted with silicon tetrachloride to form spherical support particles which were then reacted with titanium tetrachloride and diisobutylphthalate exhibited both low activity and low stereospecificity in propylene polymerization. A possible way to improve a such a catalyst would be to include a Lewis base during formation of a chlorinated support and prior to introduction of titanium tetrachloride. Among possible bases, aromatic esters, specifically ethylbenzoate, have almost universally been found to be the best compounds for this purpose. However, the use of such aromatic ester is incompatible with use of a magnesium alkyl. Usual aromatic esters such as ethylbenzoate, ethyl p-anisate, methyl-p-toluate or dialkylphthalates react rapidly and irreversibly with magnesium alkyls through alkylation of the esters. Also, it has been found that the precipitation reaction with silicon tetrachloride is disrupted and the particle uniformity or particle morphology of a resultant catalyst is destroyed.
Use of magnesium alkyls in preparation of supported olefin polymerization catalysts is known. However, formation of a stable magnesium alkyl complex with a hindered ester prior to precipitation has not been described. Examples of use of magnesium alkyls are U.S. Pat. Nos. 4,115,319, 4,199,473, 4,321,347 and 4,416,799; U.K. Pat. Nos. 1,586,267 and 2,018,789; Published European Patent Applications Nos. 45,533 and 67,416; and French Pat. No. 2,529,209.
Catalyst components of the present invention are formed using stable complexes of a magnesium alkyl composition and an alkyl hindered aromatic acid ester. Although not describing such donor-acceptor complexes of the present invention, French Pat. No. 2,529,209 discloses a catalyst component prepared from spherical magnesium chloride made by chlorinating di-n-butyl magnesium in isoamyl ether with t-butylchloride. Although we have observed that addition of specific amounts of a base such as isoamyl ether or 2,2,6,6,-tetramethylpiperidine to a magnesium alkyl-aluminum alkyl composition prior to reaction with silicon tetrachloride improves overall performance of a resultant catalyst without destroying morphology, catalysts formed using the ortho disubstituted hindered aromatic acid esters of this invention achieve a much greater improvement in catalyst performance.